EFFECTS OF WATER-SUPPLY ON PARTITIONING OF DRY-MATTER BETWEEN ROOTS AND SHOOTS IN LOLIUM-PERENNE

When plants of L. perenne were subjected to 1 cycle of soil drying, to repeated cycles of drying, or when they were grown in dry soils, the relative growth rate of the root system was reduced more than that of the shoot system. This brought about a decrease in the root:shoot ratio. Drought initially resulted in lower availability of water near to the soil surface than at greater depths. Consequently, growth of that part of the root system near the soil surface was restricted, and newly initiated main roots at the base of the shoot were unable to elongate. When plants were grown in solutions of low osmotic potential, there was no effect on distribution of dry matter between roots and shoots, except at the lowest potential (-9.5 bars) which reduced root growth rather than shoot growth. The effect of drought on root:shoot ratio in these experiments is compared with that reported by others. In some circumstances, the availability of P in the soil may influence the effect of drought on root:shoot ratios.